Chemical looping is a recently developed process which can be utilized in electrical power generation plants which burn fuels such as coal, biomass, and other fuels. The chemical looping process can be implemented in existing or new power plants, and provides promising improvements in terms of reduced plant size, reduced emissions, and increased plant operational efficiency, among other benefits.
The FIG. 1 depicts a chemical looping system 2 that comprises an oxidizer 4 and a reducer 6. In the oxidizer 4, a solid oxygen carrier such as calcium sulfide (CaS) or a metal (denoted as “Me”) are oxidized with oxygen derived from air. For example, calcium sulfide is oxidized in the oxidizer 4 to calcium sulfate. The oxygen depleted air, containing primarily nitrogen, small amounts of oxygen and other gas species, is released from the oxidizer as exhaust. The calcium sulfate is then transported to a reducer 6, where calcium sulfate is reduced to calcium sulfide with the release of oxygen. The released oxygen is used to combust a fuel supplied to the reducer 6. The combustion of the fuel in the reducer 6 produces primarily carbon dioxide, small amounts of water and other gas species (exhaust gases). The reduced calcium sulfide from the reducer is discharged to the oxidizer 4.
In summary, a chemical looping system utilizes a high temperature process, whereby solids such as calcium- or metal-based compounds are “looped” between a first reactor, called an oxidizer (or an air reactor), and a second reactor, called a reducer (or a fuel reactor). In the oxidizer, oxygen from air injected into the oxidizer is captured by the solids in an oxidation reaction. The captured oxygen is then carried by the oxidized solids to the reducer to be used for combustion and/or gasification of a fuel such as coal, for example. After a reduction reaction in the reducer, the solids, no longer having the captured oxygen, are returned to the oxidizer to be oxidized again. This cycle is repeated.
In the chemical looping system, the gas leaving the oxidizer comprises primarily nitrogen with small amounts of oxygen and other gas species, and the gas leaving the reducer comprises primarily carbon dioxide with small amounts of water and other gas species. The exhaust gas from the oxidizer is vented into the air after its heat energy is utilized. The exhaust gas from the reducer is sent to a gas processing unit for further clean up and finally becomes high purity carbon dioxide.
Since the oxidizer is fluidized with air and the reducer is fluidized with recirculated high concentration carbon dioxide, it is desirable to preheat the air with recovery heat from both exhaust gas streams. An air preheater is used to preheat the air supplied to the oxidizer with heat obtained from the exhaust gas streams.
There are different types of air preheaters. Plate and tubular type air preheaters do not have leakage between the air side and the gas side but are normally used in smaller applications due to their low heat recovery efficiency. The rotary regenerative type air preheaters, with their high heat recovery efficiency, are used dominantly in utility scale power plants. However, leakage in such rotary regenerative air preheaters is inevitable. Special design measures are required to minimize the leakage.
FIGS. 2A and 2B generally depict a conventional air preheater 10, and more particularly, a rotary regenerative air preheater 10. The air preheater 10 has a rotor 12 rotatably mounted in a housing 14. The rotor 12 includes partitions 16 extending radially outward from a rotor post 18 toward an outer periphery of the rotor 12. The partitions 16 define compartments 20 therebetween for containing heat exchange element basket assemblies 22. Each heat exchange basket assembly 22 has a predetermined effective heat transfer area (typically on the order of several thousand square feet) of specially formed sheets of heat transfer surfaces, commonly referred to as heat exchange elements 42.
In the conventional rotary regenerative air preheater 10, a flue gas stream 28 and a combustion air stream 34 enter the rotor 12 from respective opposite sides thereof, and pass in substantially opposite directions over the heat exchange elements 42 housed within the heat exchange element basket assemblies 22. More particularly, a cold air inlet 30 and a cooled flue gas outlet 26 are disposed at a first side of the heat exchanger (generally referred to as a cold end 44), while a hot flue gas inlet 24 and a heated air outlet 32 are disposed at a second side, opposite the first side, of the air preheater 10 (generally referred to as a hot end 46). Sector plates 36 extend across the housing 14 adjacent to upper and lower faces of the rotor 12. The sector plates 36 divide the air preheater 10 into an air sector 38 and a flue gas sector 40.
The arrows shown in FIGS. 2A and 2B indicate a direction of travel of the flue gas stream 28 and the combustion air stream 34 through the rotor 12, as well as a direction of rotation of the rotor 12. As shown in FIGS. 2A and 2B, the flue gas stream 28 enters through the hot flue gas inlet 24 and transfers heat to the heat exchange elements 42 in the heat exchange element basket assemblies 22 mounted in the compartments 20 positioned in the flue gas sector 40. The heat exchange element basket assemblies 22, heated by the heat transferred from the flue gas stream 28 are then rotated to the air sector 38 of the air preheater 10. Heat from the heat exchange element basket assembly 22 is then transferred to the combustion air stream 34 entering through cold the air inlet 30. The flue gas stream 28, now cooled, exits the preheater 10 through the cooled flue gas outlet 26, while the combustion air stream 34, now heated, exits the preheater 10 through the air outlet 32.
Referring to the FIG. 2C, it can be seen that the rotor 12 is dimensioned to fit within an interior of the housing 14. However, an interior void 95 is formed by spaces between the rotor 12 and the housing 14. Due to a pressure differential between the hot flue gas inlet 24 and the heated air outlet 32, a portion of the combustion air stream 34 in the air sector 38 (FIG. 2B) passes over into the flue gas sector 40 (FIG. 2B) of the air preheater 10 via the interior void 95, thereby contaminating the flue gas stream 28 with air. More specifically, and as shown in FIG. 2D, a portion of the combustion air stream 34 flows from the air sector 38 to the flue gas sector 40 along a first path LG1. In addition, portions of the flue gas stream 28 bypass the rotor 12 by flowing along a second path LG2 from the hot flue gas inlet 24 directly to the cooled flue gas outlet 26 via the interior void 95, thus decreasing an efficiency of the air preheater 10. Likewise, other portions of the combustion air stream 34 bypass the rotor 12 by flowing along a third path LG3 from the cold air inlet 30 directly to the heated air outlet 32 via the interior void 95, further decreasing the efficiency of the air preheater 10.
Leakage of the combustion air stream 34 from the air sector 38 to the flue gas sector 40 along the first path LG1 (generally referred to as air leakage) causes flue gas volume in a power plant exhaust flow to increase. As a result, a pressure drop in equipment downstream from the air preheater 10 increases, thereby increasing auxiliary power consumption in components such as induced draft (ID) fans (not shown). Likewise, increased flue gas volume due to air leakage increases size and/or capacity requirements for other power plant components, such as wet flue gas desulfurization (WFGD) units (not shown) or other flue gas clean-up equipment, for example. As a result, costs associated with power plant construction, operation and maintenance are substantially increased due to air leakage.
Moreover, in a power plant equipped with a gas processing unit for carbon dioxide (CO2) capture (not shown), leakage reduction is even more beneficial. For example, when designing the gas processing unit, air leakage needs to be taken into account. Oversizing the gas processing unit to accommodate the air leakage is expensive. Additionally, the gas compressors in the gas processing unit need to compress the increased gas flow due to the air leakage and this further increases auxiliary power requirements.
In light of the abovementioned problems associated with the conventional air preheater 10, steps have been taken in attempts to reduce air leakage, such as by using of a series of seals within the air preheater 10 to minimize leakage of the combustion air stream 34 from the air sector 38 to the flue gas sector 40. Referring to FIG. 3A, for example, a conventional air preheater 110 includes a rotor 112 mounted in a housing 114. The rotor 112 includes a rotor post 118 and is dimensioned to fit within an interior of the housing 114. In attempts to minimize air leakage, seals 220, 222, 224, 226, 228 and 230 are provided. The seals 220, 222, 224, 226, 228 and 230 extend from an interior surface of the housing 114 inward toward the rotor 112 and are positioned in spaces within an interior void 195 to reduce an amount of the combustion air stream 34 in the air sector 38 (FIG. 2B) from crossing into the flue gas stream 28 in the flue gas sector 40 (FIG. 2B). More specifically, as shown in FIGS. 3A and 3B, seals 222 and 224 define a plenum “A” which receives the flue gas stream 28 via a hot flue gas inlet 124. Similarly, seals 220 and 230 define a plenum “B” from which the flue gas stream 28, having passed through the rotor 112, is expelled via a cooled flue gas outlet 126. Further, seals 220 and 228 define a plenum “C” which receives the combustion air stream 34 via a cold air inlet 130, and seals 222 and 226 define a plenum “D” from which the air stream 34, having passed through the rotor 112, is expelled via a heated air outlet 132. Seals 220 and 222 also define a plenum “E”, while seals 224 and 226 define a plenum “F”. Seals 228 and 230, having the rotor post 118 disposed therebetween, also form a plenum “G”, as shown in FIGS. 3A and 3B.
Thus, in an effort to reduce air leakage, the conventional air preheater 110 includes the seals 220, 222, 224, 226, 228 and 230. Air heater leakage is due in large part to deflection of the rotor after it has been heated from cold to hot conditions. A hot end of the rotor deflects axially more than a cold end thereof, and therefore, gaps between the seals are different, contributing to leakage, e.g., from plenums “D” and/or “C” to plenums “A” and/or “B”, respectively, via plenums “F” and/or “G”, respectively. Air leakage, e.g., along the first path LG1 (FIG. 3C), will now be described in further detail with reference to FIG. 3D.
FIG. 3D is a top plan view of a conventional tri-sector regenerative air preheater 310. In the tri-sector regenerative air preheater 310, seals 332, 334 and 336 are provided and divide an interior of the air preheater 310 into three plenums 360, 362 and 364. Specifically, plenum 360 is a primary air (PA) plenum 360, and generally has the highest pressure level of the three plenums 360, 362 and 364. Plenum 362 is a secondary air (SA) plenum 362 and generally has the second highest pressure level of the three plenums 360, 362 and 364, while plenum 364 is a flue gas (FG) plenum 364 and has the lowest pressure level of the three plenums 360, 362 and 364. Thus, a pressure in the PA plenum 360 is greater that pressures in both the SA plenum 362 and the FG plenum 364, while a pressure in the SA plenum 362 is greater than the pressure in the FG plenum 364 but less than the pressure in the PA plenum 360, and the pressure in the FG plenum 364 is less the pressures of both the PA plenum 360 and then SA plenum 362.
In another conventional quad-sector regenerative air preheater (not shown), seals are provided and divide an interior of the air preheater into four plenums. The PA plenum generally has the highest pressure level of the four plenums. The SA plenums having equal pressures (and generally the second highest pressure level of the four plenums while the FG plenum has the lowest pressure level of the four plenums.
In FIG. 3D, broken arrows (labeled “Flow”) depict flow of gases from plenums at higher pressure into plenums at relatively lower pressures. Specifically, in the conventional tri-sector regenerative air preheater 310, air leakage occurs from both the PA plenum 360 and the SA plenum 362 into the FG plenum 364, as shown in FIG. 3D. Likewise, in the conventional quad-sector regenerative air preheater, air leakage occurs from both SA plenums and into the FG plenum. In summary, the aforementioned preheater comprises four sectors (plenums), where the flue gas flows through the largest sector, while the primary air and secondary air travel through three other smaller sectors.
Despite the use of seals, air leakage still occurs in a conventional air preheater, despite the addition of seals designed to prevent the air leakage. Accordingly, it is desirable to develop an air preheater having substantially reduced and/or effectively minimized air leakage.